The present invention relates to a method for preventing wandering of a strip in a hot rolling line and, more particularly, to a method for correcting a hot rolled strip in which shape defects of the hot rolled strip are corrected in the hot rolling line and the wandering of the strip during the correction is prevented.
Demands for increased flatness of hot rolled strip have recently become more and more severe, and now, if sufficient flatness is provided, hot rolled strips can be used in place of more expensive cold rolled strips.
In the manufacture of steel strips by a continuous hot rolling mill, the relatively large distance between the final mill stand of the finishing mill and the coiler presents a problem in that the strip bottom, having passed the final finishing stand, becomes under zero tension and flutters. The fluttering of the strip bottom is a cause for such shape defects as out of flatness, edge defect, and edge break and defective appearance of the coil.
For this reason, hot rolled strips have been, after rolling in the continuous hot rolling mill, temper-rolled and simultaneously corrected in flatness. If the hot strip produced by such mill had satisfactory flatness it would be possible to eliminate the temper rolling step. Heretofore, however, it has been necessary to conduct the temper rolling operation to obtain the required flatness of the hot rolled strips.
As mentioned above, the insufficient flatness of the hot rolled steel strips causes defective appearance of the coil which presents noticeable problems in the succeeding production steps such as, for example in the case of mother material for cold rolling, decrease in pickling efficiency resulting from decrease in passability of plates in the pickling line and wandering of the strip.
To overcome these problems, the inventors previously presented a method in which a hot roller leveler is provided between the final finishing mill and the coiler, so that the flatness of the hot rolled steel strip is corrected by the roll force of the hot roller leveler and the tension between the finishing mill and the coiler (Japanese Patent Publication No. 48182/78 which issued as Japanese Patent No. 968053).
It has been confirmed that such previous method mentioned above sharply reduced the shape defects of strips such as center buckles and edge waves. However, in the cases where the thickness distribution of the strip in the widthwise direction is not symmetrical, where the rolled shape is extremely asymmetrical, where the temperature distribution of the strip is asymmetrical (mainly due to unsatisfactory spray), and where the parallelism among the units of the rolling line is not complete (it is difficult to maintain complete parallelism among the units throughout the long rolling line), the leveling of the strip by the hot roller leveler sometimes caused the strip to wander sideways so violently that the strip edge hit the support members (side guides, etc.) to thereby cause edge defects and, in the extreme case, make the coiling operation impossible.